"Clean up your mess behind you". Always try not to leave the mess behind you in the kitchen and the bathroom. Small things that happen over and over can easily become the last drop.
For the big cleanup operations of common apartment parts, it's best to organize group actions or let every roommate do them in their own turn.
Try to pay the rent and the bills in time. A roommate you owe to can become dangerous :)
Try to avoid surprise visits that could bother your rommates - a party for 50 people in one room while the roommate is trying to prepare for an exam in the room beside can easily be a threat for good roommate relations.
The most frequent problem is boyfriend's or girlfriend's visits being so frequent that they become your new roommates. If the room If the roommates mind that, you should avoid to be in that situation, or event better: welcome the boyfriend or girlfriend and accept them as a new roommate - you will reduce your expenses and keep a good atmosphere in the apartment.
It's very important that everyone has his or her own private space (it would be best if everyone had a separate room). The respect of privacy is necessary for good relations.
If one roommate's room is smaller then the others, try to make some kind of compensation: you can switch rooms every few months, make the roommate with the smallest room pay less for the rent, let him or her use a closet in the commun rooms, etc.
It's very important that every roommate feels equal.
If you're sharing a phone line, get a detailed call list. If every person pays what he or she spends, there's no place for conflicts.
Always try to have at least a minimum of respect for your roommates: knock before entering their room, don't enter their room while they're not there, don't test the sound system power while they're sleeping, and don't make rave parties in the apartment while they're studying. Just try not to do anything you wouldn't like them do to you.
Try not to choose roommates that are too different from you. You will organize your everyday life with your roommates, so if they are too different or they have different definitions for privacy and respectthe everyday life will probably be hell...
As yourself, your roommate has some good and some bad moments. In those situations, it's important to be tolerant.
Try to buy some basic necessary things in common - you can make some important savings in your monthly budget that way!